A very significant limitation on the operation and reliability of semiconductor chip packages is the efficient extraction of heat. It is desirable to provide the packages, particularly high power packages, with an efficient heat transfer mechanism, such as heatsinks, to maintain them within a predetermined operating temperature range. It is a problem to reliably bond aluminum heatsinks to packages, specifically if the packages are made of, for example, a ceramic.
Heatsinks are normally bonded to semiconductor chip packages using an adhesive, typically an epoxy. In order for the epoxy to achieve the desired bonding strength to secure the heatsinks to the packages, pressure and heat are normally used during the bonding process. Pressure is used to attain the desired bond-line thickness, and heat is used to cure the adhesive.
In a known system (pre-reflow attachment), the adhesive, and sometimes an accelerator, are dispensed onto a heat removal surface of the semiconductor chip packages prior to assembling the packages onto a printed circuit board by, for example, soldering. The adhesive is usually spread on the package with a roller or dispensed in a dot pattern, or alternatively, deposited as a pre-formed film. The heatsinks are then pressed to the packages to obtain the desired bond-line thickness, typically by means of pneumatically operated pressure devices or springs. The packages with the heatsinks attached are cured in batches in a convection oven at a temperature between 150 and 180 degrees C. for thirty minutes or more. Adhesive films require that pressure be applied throughout the entire cure cycle. Subsequently, the packages, with the heatsinks attached, are assembled onto the printed circuit board by, for example, reflowing solder.
However, semiconductor chip packages with heatsinks attached are difficult to handle with automated insertion machines, such as vacuum pick-and-place tools. Planarity problems, due to misalignment between the heatsinks and the packages, and surface irregularities of the heatsinks, interfere with the insertion of the packages on the printed circuit board by the vacuum tools. In addition, as the size and power of the packages are increased so is the size of the heatsinks. The more massive packages and heatsinks generally require more complex and expensive pick-and-place tools. Also, as the size of the bonding area is increased with larger heatsinks, ceramic packages are more likely to be stressed to the point of fracture during the solder reflow process when the packages are connected to the printed circuit board.
Therefore, in an alternative system (post-reflow attachment), the heatsinks are attached to the semiconductor chip packages after the packages have been assembled onto the printed circuit board. In this system, the accelerator and the adhesive are dispensed onto the packages after the packages have been connected to the printed circuit board. This system has the disadvantage that a second heating cycle in the convection oven at a high temperature can not be used to cure the adhesive. Heating the adhesive to a high temperature may also cause the solder to reflow to disconnect the packages from the printed circuit board. Therefore, the adhesive is cured at room temperature, by way of example, for as long as a day. It is a problem to provide a constant pressure to the adhesive when the time for curing is extended.
Accordingly, the known systems for bonding heatsinks to semiconductor chip packages increase the cost of assembly, or do not always provide for efficient or reliable bonding.
Therefore, it is desirable to provide a system for reliably bonding heatsinks to semiconductor chip packages which is simple and uses readily available inexpensive materials and assembly equipment, and which is easily adaptable to mass production methods at reduced costs.